The invention is based on an electrochemical measuring probe for determining the oxygen content of gases as defined by the generic class of the main claim.
To determine the oxygen content of gases, in particular of exhaust gases of internal combustion engines, it is generally known, for example from German Patent Specifications 2,206,216, 2,852,638 and 2,619,746 and from German Offenlegungsschrift 2,913,633, to use electrochemical measuring probes--often also referred to as gas sensors--comprising a solid electrolyte, for example stabilised zirconium dioxide, a first electrode on the side of the solid electrolyte exposed to the gas to be measured, a second electrode on the side of the solid electrolyte exposed to a comparison gas of known oxygen content and a porous ceramic protective layer on the first electrode, which is exposed to the gas to be measured.
From German Patent Specification 2,852,638 and German Offenlegungsschrift 2,913,633, it is furthermore known to use, as electrodes exposed to the gas to be measured, electrodes composed of a finely divided ceramic material to form a supporting matrix and a finely divided electron-conducting material, so-called cermet electrodes. In this connection, the finely divided ceramic material used to form the supporting matrix may consist, for example, of stabilized zirconium dioxide or aluminium oxide, and the finely divided electron-conducting material, for example, of platinum, a platinum alloy, for example platinum-rhodium alloy, or a palladium-noble metal alloy.
From German Patent Specification 2,852,638, it is furthermore known to produce the measuring probe body from partially stabilized zirconium dioxide and the supporting matrix of the cermet electrode from fully stabilized zirconium dioxide.
From German Offenlegungsschrift 3,737,215, it is furthermore known to deposit a porous ceramic protective layer composed of an Al.sub.2 O.sub.3 -- and/or Mg-spinel (MgO.Al.sub.2 O.sub.3)-matrix with ZrO.sub.2 particles included therein on the electrode exposed to the gas to be measured.
Finally, from German Offenlegungsschrift 3,735,298 it is known to deposit a densely sintering covering layer composed of the raw material mixture of the probe-base ceramic having the same or increased sintering activity on the supply lead of the electrode, exposed to the gas to be measured, of an electrochemical measuring probe having a probe base formed from a solid electrolyte. This covering layer has the advantage that it can be applied to the conductor track before the sintering process and screens the conductor track so hermetically that a trouble-free continuous operation of the measuring probe is possible even in very rich exhaust gases (.lambda..ltoreq.0.8) at high temperatures with heavy soot formation.